It is known to manufacture a synthesis gas formed of hydrogen and carbon oxides, by controlled oxidation of hydrocarbons mixtures such, for example, as natural gas, liquified petroleum gas or naphtha.
The operation is conducted at high temperature and in the presence of steam.
From a theoretical point of view, the controlled oxidation is of particular interest for the synthesis gases intended for the manufacture of methanol, since it makes possible, by adjustment of the oxygen amount, to obtain a gas having exactly the required stoichiometry for producing said alcohol.
As a matter of fact, for example, the following equations can be written for the first members of saturated hydrocarbons: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+2H.sub.2 ( 1) EQU C.sub.2 H.sub.6 +1/2O.sub.2 +H.sub.2 O.fwdarw.2CO+4H.sub.2 ( 2) EQU C.sub.3 H.sub.8 +1/2O.sub.2 +2H.sub.2 O.fwdarw.3CO+6H.sub.2 ( 3)
More generally, equation is: EQU C.sub.n H.sub.2n+2 +1/2O.sub.2 +(n-1)H.sub.2 O.fwdarw.nCO+2nH.sub.2 ( 4)
Methanol syntheses proceed according to the equation: EQU nCO+2nH.sub.2 .revreaction.nCH.sub.3 OH (5)
In fact, the reactions are slightly more complex since at the temperatures at which the synthesis gas is manufactured, water reacts as well with hydrocarbons as with CO according to the well known water gas formula: EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 ( 6)
The CO.sub.2 content of the synthesis gas is then used to produce methanol according to equation: EQU nCO.sub.2 +3nH.sub.2 .revreaction.nCH.sub.3 OH+nH.sub.2 O (7)
The safer and more common oxidation method is vivid flame combustion, either with air or with pure oxygen.
Pure oxygen is used for manufacturing synthesis gases, except for the manufacture of ammonia, where nitrogen is a necessary reactant.
The vivid combustion with pure oxygen, however suffers from a number of disadvantages.
On the one hand, numerous difficulties arise with respect to the resistance of the materials to the very high temperature of the flame, particularly when proceeding under high pressure.
On the other hand, when the oxygen amount is insufficient to stoichiometrically convert carbon to carbon dioxide and hydrogen to steam, the combustion is incomplete and the operation always results in a more or less substantial formation of soot (C. P. Marion & J. R. Muenger, AICHE Meeting, 5-9 April 1981).
For this reason, in the usual partial oxidation processes, the ratio of oxygen molecules to the carbon atoms to be burnt must not be lower than 0.7 (Hydrocarbon processing, September 1979, p. 191-194).
Now, when comparing equations (1) to (4), it is observed that the O.sub.2 /C ratio ranges from 0.5 for equation (1) to 1/2n for equation (4), n being the number of carbon atoms of the molecule to be oxidized.
Another method for manufacturing synthesis gas is the catalytic oxidation of hydrocarbons.
This method is commonly used under atmospheric pressure in industrial processes such as ONIA-GEGI process, for example.
The increasing cost of the power required for compressing the manufactured gas makes these processes rather uneconomical.
In the sixties, tests have been performed in order to operate the catalytic oxidation under pressure, which is the only way of complying with the stoichiometry for methanol production.
When performed with pure oxygen, these tests were unsuccessful either because of the explosion phenomena, or because of clogging phenomena of the catalyst bed (Chem. Eng. Prog., Vol. 61, no 11, p. 85-88, November 1965).
When performed with mixtures of air and enriched air, these tests have led to the so-called primary and secondary reforming processes (D. R. Holland & S. W. Wan, Chem. Eng. Prog. Vol. 55 (8), p. 69-74, August 1963) used in ammonia synthesis.
In these processes, in order to avoid the clogging phenomena, a portion or the totality of the hydrocarbons charge is subjected to steam reforming before contacting the whole charge with oxygen diluted with nitrogen of air.
In order to maintain the oxygen concentration below a certain limit, particularly when the hydrocarbons are of the naphtha type or heavier, it has been proposed (U.S. Pat. No. 3,278,452) to introduce the nitrogen and oxygen mixture in 2 successive steps.
Recently, it has been proposed to apply the method of primary and secondary reformings to pure oxygen (French Pat. No. 2 372 116).
A comparison of the oxygen content of the mixture with the limits of explosiveness of gases in pure oxygen, makes obvious the potential difficulties arising for such an application.